Process for preparing microspheroidal silica



United States Patent 3,453,077 PROCESS FOR PREPARING MICROSPHEROIDALSILICA James R. Hyde, Cincinnati, Ohio, assignor to W. R. Grace & Co.,New York, N.Y., a corporation of Connecticut No Drawing.'Continuation-in-part of application Ser. No. 453,186, May 4, 1965. Thisapplication Aug. 10, 1967, Ser. No. 659,577

Int. Cl. C01b 33/16 US. Cl. 23-482 4 Claims ABSTRACT OF THE DISCLOSUREThis application is a continuation-in-part of application Ser. No.453,186 filed May 4, 1965 now abandoned.

In summary, the process of this invention comprises the steps ofpreparing an alkali metal silicate solution having a silicate content,expressed as silica, of from 3 to 12 percent, gelling the silicate withgaseous carbon dioxide, aging the gelled silicate at a temperature offrom 70 to 180 F. for a period of from 0 to 3 hours, mixing the hydrogelwith a quantity of a mineral acid suificient to adjust the pH of thehydrogel to less than 7.0, aging the acidified hydrogel at a temperatureof from 70 to 180 F. for a period of from 0 to 3 hours, mixing thehydrogel with a quantity of ammonia sufficient to raise the hydrogel pHup to from about 7 to 9.5, aging the hydrogel for a period of from 0 to6 hours at a temperature Within the range of 70 to 180 F., spray dryingthe hydrogel, washing the spray dried gel to remove soluble impurities,and redrying the silica gel. In a further embodiment of my invention,the initial gela-tion may be effected using an acid, such as sulfuricacid, rather than carbon dioxide and then treating the gel in the samemanner as outlined above.

It is known that almost all solids are capable of adsorbing gas, vapors,or liquids on their surface to some degree and that the amount adsorbeddepends on the nature and extent of the surface. One of the better knownadsorbents is commercial silica gel which has a lattice structure formedby an extensive network of void spaces. This structure permits the gelto adsorb approximately 50 percent of its weight of water from saturatedair. From this, it has been estimated that one cubic inch of silica gelcontains pores having a surface area of about 50,000 square feet. Withthis enormous internal surface and infinite number of small diametercapillaries, silica gel attracts vapors, condensing them and holdingthem as liquids. The same phenomenon applies to the adsorption ofliquids.

The silica gels are normally prepared by mixing a sodium silicatesolution with a quantity of an acid sufficient to completely neutralizethe sodium silicate solution. This step produces a hydrosol, which uponstanding forms a hydrogel. The hydrogel, when aged, undergoes syneresis.This phenomenon shrinks the gel. The beginning of syneresis of thesilica gel can be detected by the appearance of droplets of water on thesurface of the gel.

After the beginning of syneresis, the silica gel is washed. The methodof washing the gel determines the type of gel to be recovered as theproduct. If a regular density gel is to be recovered, the gel is Washedwith a dilute 3,453,077 Patented July 1, 1969 sulfuric acid solution.After the washing is complete, the gel is dried. The regular densitysilica gel normally has a surface area of about 800 m. /g. and a porevolume of 0.3 to 0.4 cc./ g. If a silica gel product is desired having asurface area of about 300 to 500 m. /g., the gel is washed with anaqueous ammonia solution at the outset of syneresis. The productrecovered from this treatment has a surface area of about 300 to 500mfi/g. and a pore volume of 0.65 to 1.0 cc./g. If a gel having a surfacearea of from 500 to 800 m. /g. is desired, the gel is washed with waterat the beginning of syneresis. The product recovered has a pore volumeof from about 0.35 to about 0.65 cc./g.

Silica gels are widely used as catalyst bases, fillers, desiccants, etc.Development of new industrial processes and the refinement of oldprocesses have created needs for silica gels having surface area andpore volume combinations which cannot be economically produced by theabove methods. A combination of a particular pore volume and surfacearea range which falls outside of those normally produced by theabove-described conventional processes is often desired. However, itshould be stressed that my invention resides not in the product but in anew and unobvious process of preparing such silica gels. For example, incertain polymerization processes, silica gels with a pore volume greaterthan one cc./ g. but with a surface area which is within the range offrom to 600 m. g. are required as catalyst bases.

It is the object of this invention to provide practical, economicalprocesses for forming silica gels having pore volumes within the rangeof from 1.0 to 2.5 cc./ g. and surface areas within the range of from100 to 600 m. g.

Unless otherwise stated, concentrations are herein given as weightpercents.

The first step of the process of this invention comprises preparing analkali metal silicate solution having a silicate content, expressed assilica, of from about 3 to 12 percent. The preferred silica level isbetween 4.5 and 9 percent silica. Suitable alkali metal silicatesinclude sodiurn silicate and potassium silicate. The desiredconcentrations can be obtained by dilution of more concentrated silicatesolutions. The silica to alkali metal oxide weight ratio of the alkalimetal silicate is not critical, but the preferred range is from about3.0:1 up to 3.4: l.

The next step in the process of this invention comprises gelling thesilicate with gaseous carbon dioxide. This can be accomplished bybubbling the gas into a vessel containing the alkali metal silicate, orthe reactants can be contacted in a mixing nozzle. Regardless of themethod of mixing employed, the reactants are thoroughly agitatedfollowing mixing and during the formation of the silica hydrogel so thatan aqueous slurry of hydrogel particles containing dissolved alkalimetal carbonate is formed. The gelation with gaseous carbon dioxide canbe followed by an aging step up to 3 hours at temperatures of from 70 toP. if desired. It has been found that aging at this stage of the processeffects an increase in the pore volume and a decrease in the surfacearea of the product gel. Increased aging temperatures have been found toincrease this effect.

The next step in the process comprises mixing the hydrogel with aquantity of a mineral acid sufficient to adjust the pH of the gel toless than 7.0. This acidification step provides increased neutralizationof the alkali metal silicate and releases carbon dioxide from the gelslurry. The carbon dioxide can be collected for reuse in the process.The acid treatment of the hydrogel can be followed with an aging step,if desired. It has been found that aging at this stage of the processalso effects an increase in pore volume, however, to a lesser extentthan aging immediately following the gelation step.

The next step in the process of this invention comprises mixing thehydrogel with a quantity of ammonia, preferably an ammonium hydroxidesolution, sufficient to raise the gel pH to from above 7 to 9.5. Thealkaline treatment for pH adjustment can be followed with an aging stepcomprising aging the hydrogel for from to 6 hours at a temperaturewithin the range of 70 to 180 F. It has been found that ammoniation ofthe silica hydrogel at this stage of the process effects a decrease insurface area of the silica gel while at the same time producing only aslight change in the pore volume as contrasted with the prior arttailoring processes in which a substantial decrease in surface arearesults in a substantial increase in pore volume. In my process I amable to decrease surface area with no or only a very slight increase inpore volume and in some cases actually effect a decrease in both porevolume and surface area. Adjustment to the higher pH values, increasedaging times, and increased aging temperatures have been found toincrease this effect.

In the next step of the process the silica gel is spray dried. Thematerial can be filtered before it is spray dried but, however, noapparent quality advantage is derived from the filtration and thematerial can be spray dried directly. In spray drying, the silica gel issprayed as a fine mist into a drying chamber heated by a suitable means,such as combustion of propane. The mist is introduced into the top of aspray drier where it contacts a countercurrent flow of hot gases in itsdownward flight and settles to the bottom as solid, dry, spheroidalparticles of substantially uniform size and shape. Control of particlesize may be effected by controlling the characteristics of both thenozzle of the spraying device and the slurry being dried. The spraydrier is commonly operated with an inlet temperature of from 800 to 1000F. and an outlet temperature of about 300 to 400 F.

The spray-dried silica gel is then washed to remove soluble impurities.The washing is carried out with a dilute mineral acid solution having anormality of less than N. Aqueous sulfuric acid solutions havingconcentrations within the range of from 0.2 to 5 percent can beemployed. The wash is preferably carried out at elevated temperatureswithin the range of from 100 to 140 F. Particularly satisfactory resultsare obtained when the material is washed with a dilute sulfuric acidsolution having a concentration range of from 0.5 to about 2 B. Thesilica gel, alternatively, can be washed with a dilute ammonium sulfatesolution having a concentration of from 0.5 to 5 percent ammoniumsulfate. After the acid Wash, the product can be washed with deionizedwater to remove the residual soluble impurities from the gel. After thematerial is washed, it is dried at temperatures of from about 250 to 500F. The time of drying is adjusted so that the final product has a totalvolatile content, principally moisture, of about 4 to percent.

In an alternative embodiment of my invention, the initial gelation iseffected by using a mineral acid rather than carbon dioxide, though, ofcourse, the carbon dioxide embodiment is economically more preferable.The first steps of the acid process are similar to conventional acid gelpreparations and comprise first preparing an aqueous alkali metalsilicate solution containing from about 6 to 12 percent silicate,expressed as silica. Suitable alkali metal silicates include sodiumsilicate, potassium silicate, and the like. The alkali metal silicatesolution is heated to a temperature within the range of from about 110to 170 F. and preferably from about 120 to 160 F.

The next step in the process of this embodiment comprises mixing thealkali metal silicate solution with a quantity of a mineral acid whichis sufiicient to effect substantially 100 percent neutralization of thealkali metal silicate. Suitable mineral acids which can be present inthe acid solution include hydrochloric acid, nitric acid, phosphoricacid, sulfuric acid, and the like. Sulfuric acid is preferred foreconomic reasons. Sulfuric acid solutions which are suitable can haveconcentrations within the range of from 20 to 50 percent and preferablywithin the range of from 35 to 45 percent. The acid solution and thealkali metal silicate solution are mixed with stirring. The addition ofthe acid solution to the alkali metal silicate solution is usually madeover a period of from about 5 to 30 minutes, and preferably from about10 to 15 minutes. Gelation normally occurs in less than 6 minutes.Agitation and recirculation of the mixture are maintained throughoutgelation and aging.

The silica gel is then aged at a temperature of from about to 190 F. forfrom 0.25 to 6 hours, preferably 0.5 to 3 hours. The pH is then checkedto ensure that it is not above 7.0, and if above 7.0 sufiicient acid isthen added to reduce the pH to below 7.0 to ensure essentially completeneutralization of the alkali metal silicate. The remainder of thetreatment which is the essence of my invention is the same as set forthabove with respect to the carbon dioxide gelling embodiment, Column 3,lines 1 through 55.

The product of my invention has a surface area within the range of from100 to 600 m. /g. and a pore volume greater than one cc./ g. The productsilica gel is very useful as a base in the preparation of catalysts suchas polyolefin polymerization catalysts.

The invention is further illustrated by the following specific butnonlimiting examples.

EXAMPLE I This example shows how the process of this invention can beemployed to provide a silica gel having a high pore volume and anintermediate surface area.

A dilute sodium silicate solution containing 8.5% silicate, expressed assilica, was prepared. This solution was pumped at a rate of one gallonper minute through a reaction coil, and carbon dioxide was added so thatgelation occurred within 40 to 60 seconds. The mixture having atemperature of F. was pumped for 30 minutes into an agitated tank. Theresultant hydrogel slurry was heated to 180 F. over a period of 35minutes, and was then aged at this temperature for 1.5 hours. The pH ofthe aged gel was 10.6. A quantity of a 39% sulfuric acid solution wasadded over a period of 25 minutes, sufficient to lower the gel pH to4.9. Then the acidified gel was mixed with a 23% ammonia solution in aquantity sufficient to increase the gel pH to 7.8. The gel was aged for3 hours at 180 F. The gel was spray dried, washed with a 1 B. sulfuricacid solution having a temperature of 135 F., and was then washed withdeionized water having a temperature of 135 F. The product was dried at400 F.

The product was analyzed for sodium and sulfate contents using standardanalytical procedures. The surface area of the product was determinedusing the well-known Brunauer-Emmett-Teller method. The pore volume ofthe gel product was determined using standard techniques, as describedin Analytical Chemistry, Innes, W. B., (28, 332- 4, 1956). The producthad a surface area of m. /g., a pore volume of 1.32 cc./g., a sodiumcontent (expressed as the oxide on a dry basis) of 0.03%, and a sulfateion content of 0.29%.

EXAMPLE II This example shows how the process of this invention can beemployed to provide silica gels having both surface areas and porevolumes greater than those obtained following the procedure of ExampleI.

A sodium silicate solution containing 6.1% silicate, expressed assilica, was prepared. The silicate solution was pumped at a rate of onegallon per minute through a reaction coil, and carbon dioxide was addedso that gelation occurred within 45 to 60 seconds. The pH of the gel was10.35. The gel, having a temperature of 130 F., was pumped for 30minutes into an agitated tank. The gel was heated over a period of 30minutes to 180 F.

and aged for one hour at this temperature. The aged gel was mixed with aquantity of 25% sulfuric acid solution over a period of 25 minutes,suflicient to reduce the gel pH to 4.7. The acidified gel was aged for25 minutes. The gel was then mixed with a quantity of 23% ammoniasolution suflicient to raise the gel pH to 8.0, and the gel was aged for3 0 minutes at a temperature of 180 F. The gel was then spray dried,washed with a 1 Be. sulfuric acid solution having a temperature of 135F., then washed with deionized water having a temperature of 135 F., andwas then dried at 400 F.

The product gel was then analyzed as described in Example I. The productgel had a surface area of 308 m. /g., a pore volume of 1.92 cc./g., asodium content (expressed as the oxide) of less than 0.05%, and asulfate content of less than 0.5% on a dry basis.

EXAMPLE III This example illustrates the advantageous effect of myprocess as contrasted with the same identical process but Without myammoniation step. In this example a silica hydrogel was prepared insubstantially the same manner as in Example I and then split into twoportions, one portion being treated in accordance with my invention andone portion being treated without my ammoniation step.

More specfically, a dilute silicate solution containing 8.4% silicate,expressed as silica, was prepared. This solution was pumped at a rate ofone gallon per minute through a reaction coil and carbon dioxide wasadded to that gelation occurred within 40-50 seconds. The mixture havinga temperature of 135 F. was pumped for 30 minutes into an agitated tank.The resultant hydrogel slurry was heated to 180 F. over a period of 30minutes, and was then aged at this temperature for 1.5 hours. A quantityof a 39% sulfuric acid solution was added over a period of 30 minutes tolower the gel pH to 4.9. At this point the batch was split into twoparts:

Part A.-The acidified hydrogel was aged three hours at 180 F., thencooled to 112 F. to retard aging until Part B was dried.

Part B.-The acidified hydrogel was mixed with sufiicient 23% ammoniasolution to increase the gel pH to 7.8. This gel was then aged threehours at 180 F.

Both portions were spray dried, washed with a 1 B. sulfuric acidsolution at 135 F. and then washed with deionized water at 135 F. Theproduct was finally dried at 400 F. and then analyzed in the same manneras Example I.

EXAMPLE IV This example illustrates the advantageous effect of myprocess as contrasted with the same identical process but without myammoniation step. In this example a silica hydrogel was prepared insubstantially the same manner as in Example II and then split into twoportions, one portion being treated in accordance with my invention andone portion being treated without my ammoniation step.

A sodium silicate solution containing 6.1% silicate, expressed assilica, was prepared. The silicate solution was pumped at a rate of onegallon per minute through a reaction coil, and carbon dioxide was addedso that gelation occurred within 40-50 seconds. The pH of the gel was10.3. The gel, having a temperature of 130 F., was pumped for 30 minutesinto an agitated tank. The gel was heated over a period of 35 minutes to180 F. and aged for one hour at this temperature. The aged gel was mixedwith a quantity of a 25% sulfuric acid solution over a period of 25minutes suflicient to reduce the gel pH to 4.7. The acidified gel wasaged for 25 minutes, then split into two parts:

Part A.-The gel was aged 30 minutes additionally at 180 F., then cooledto 112 F. to retard aging until Part B was dried.

Part B.-The gel was mixed with a quantity of a 23% ammonia solutionsufficient to raise the gel pH to 9.0, and the gel was aged for 30minutes at 180 F. Both parts were then spray dried, washed and finallydried and analyzed as in Example III.

Analysis:

Surface area, mfi/g Water pore volume, cc./g Percent NazO Percent S04Percent A 2 2 This example illustrates the embodiment of my inventionwherein the gelling agent is a mineral acid.

In this example a 20 gallon solution of dilute sodium silicatecontaining 8.1% silicate, expressed as silica, and having a temperatureof F. was prepared. To this solution was added a quantity of a 39%sulfuric acid solution sufficient to reduce the pH of the mixture to10.5 over a period of 17 minutes. Gelation occurred 7 minutes later. Thesilica hydrogel was then mixed with sufficient 39% sulfuric acid over aperiod of 70 minutes to reduce the pH of the silica gel to 5.9. Theacidified gel was then aged for 2 hours at 135 F. The acidified gel wasthen mixed with a 23% ammonia solution in a quantity sufiicient toincrease the gel pH to 8.0. The ammoniated gel was mixed 17 minutes,then spray dried, washed with a 1 B. sulfuric acid solution at 135 F.,then washed with deionized water at 135 F., and then dried at 400 F. Thesurface area and pore volume of this product were determined in the samemanner as in Example I and were found, respectively, to be 384 m. g. and1.64 cc./ g.

Obviously, many modifications and variations of the invention ashereinabove set forth may be made without departing from the essence andscope thereof, and only such limitations as are set forth in theappended claims should be applied.

I claim:

1. A process for preparing a microspheroidal silica having a surfacearea within the range of from 100 to 600 m. /g. and a pore volumegreater than one cc./g. comprising the steps of: t

(a) preparing an alkali metal silicate solution having a silicatecontent, expressed as silica, of from 3 to 12% by weight,

(b) gelling the silicate with a gelling agent selected from the groupconsisting of carbon dioxide and mineral acids,

(c) aging the gelled silicate at a temperature of from 70 to 180 F. fora period of from 0 to 6 hours,

( 1) mixing the hydrogel with a quantity of a mineral acid sufficient toadjust the pH of the gel to less than 7.0,

(e) aging the acidified hydrogel at a temperature of from 70 to 180 F.for a period of from 0 to 3 hours,

(f) mixing the hydrogel with a quantity of ammonium hydroxide solutionsufficient to raise the gel pH to the range from above 7 to 9.5,

7 (g) aging the hydrogel for 0 to 6 hours at a temperature within therange 'of from 70 to 180 E, (h) spray drying the hydrogel, (i) washingthe spray dried gel to remove soluble impurities, and (j) redrying thesilica gel. 2. The process according to claim 1 wherein the gellingagent is sulfuric acid.

3. The process according to claim 1 wherein the gelling agent is carbondioxide and wherein the gelled silicate is aged from 0 to 3 hours instep (c).

References Cited UNITED STATES PATENTS 6/1963 Jenkins et al. 2524513/1966 Carr et al. 23182 EDWARD J. MEROS, Primary Examiner.

U.S. Cl. X.R.

